The rotor blades of gas turbines, which are fastened on the rotor and exposed to the hot gas flow in the turbine, can be equipped with a shroud segment on the blade tip, which together with shroud segments of other blades of a blade row, form an annular shroud which lies concentrically to a rotor axis. As a result of the shroud, the blade row can be mechanically stabilized and a secondary flow of hot gas across the blade tip can be reduced. Therefore, aerodynamic efficiency can be increased. Such shroud segments and methods and devices for their cooling are disclosed in, for example, EP-A2-1 041 247, EP-A1-1 591 626 and GB-A-2 434 842.
Some of these shroud segments can be equipped with widened portions of a segment base in front of a first rib on a leading edge of a blade airfoil. This widened portion can be referred to as a “winglet.” Such a blade is reproduced in FIG. 1. The blade 10 of FIG. 1 includes a blade airfoil 11, which extends in a blade longitudinal direction (corresponding to the radial direction on the rotor), having a leading edge 13 and a trailing edge 12. The blade airfoil 11 terminates in a blade tip 14 and at the blade tip 14 merges into a shroud segment 16. On the upper side of the flat shroud segment 16, there are two ribs 17 and 18 which, projecting upwards, extend transversely to the flow direction of the hot gas flow 21 and together with the corresponding ribs of the other blades of a blade row form an encompassing ring in each case.
In front of the first rib 18 in the flow direction, the base of the shroud segment 16 extends forwards (upstream), forming a winglet 19 which lies in the region of the leading edge 13 of the blade airfoil 11 and towards the front is delimited by a slightly rounded leading edge 24.
The winglet 19 can prevent hot gas penetrating directly across the first rib 18 into the cavity above the shroud which is formed between the two ribs 17 and 18. Because the winglet 19 projects directly into the hot gas flow 21, it can be exposed to high temperatures. As a result of this, the material properties deteriorate and high thermal stresses occur on the winglet 19, for example, on account of the mismatch in the metal temperatures between the uncooled winglet 19 and the cooled main volume of the shroud segment 16.
Attempts have been made to reduce the temperature on the winglet by a substantial cooling air mass flow being injected into the hot gas flow 21 in the region of the blade tip 14 in order to locally reduce the temperature of the flowing medium around the winglet. This very indirect cooling, however, is effective to only a limited degree, is difficult to meter and, as a result of the comparatively large injected cooling air mass flow, impairs the efficiency of the system.